sour milk

Sour milk is milk that has undergone natural or controlled lactic acid fermentation, resulting in a tangy, acidic dairy product with a distinctive sour flavor. This fermentation occurs when lactic acid bacteria (primarily Lactobacillus species) consume the lactose in milk, producing lactic acid as a byproduct, which lowers the pH and causes the milk proteins to coagulate. Sour milk differs from spoiled milk in that the fermentation is either naturally encouraged through warmth or deliberately initiated through bacterial cultures, producing a safe, stable product with characteristic tang and slight thickening. The texture ranges from pourable to slightly viscous depending on fermentation time and temperature.

Sour milk is a traditional ingredient in many European and Eastern European cuisines, where it has been produced for centuries before refrigeration enabled the preservation of fresh milk. It remains distinct from buttermilk (a byproduct of butter churning) and yogurt, though all three share lactic acid fermentation as their basis.

Sour milk is employed across Northern and Eastern European cooking as both a culinary ingredient and a leavening agent. It features prominently in baked goods, particularly pancakes, quick breads, and cakes, where its acidity reacts with baking soda to produce lift. In Scandinavian and Russian cuisines, sour milk serves as a base for sauces, dressings, and marinades, and is drizzled over vegetable dishes or used in soups for tangy depth. It also appears in traditional breads, porridges, and slow-cooked dishes. The ingredient's acidity tenderizes meats and provides a preservative effect in traditional recipes.